Continuous casting machines which utilize at least one relatively thin flexible endless belt, have long been is use. Twin-belt continuous metal casting machines have been described generally in U.S. Pats. Nos. 2,904,860, 3,036,348, 3,041,686, 3,123,874, and 3,167,830.
The term "twin-belt casting machine" as used herein is understood to include not only machines with a straight casting section but also machines in which the two belts, normally of metal and constituting the mold, follow an arcuate path through the casting section. For example, one belt of a pair of belts may constitute the periphery of a wheel as described in prior U.S. Pat. No. 3,785,428; this results in a shape of casting path which is a sector of a circle. Or with another arrangement, the arcuate path may be of variable curvature, rather like the curve of a banana, as in U.S. Pat. No. 4,505,319 of Kimura, assigned to Hitachi.
Earlier apparatus which is relevant to the present invention is disclosed in U.S. Pat. Nos. 3,864,973 and 3,921,697, both patents being issued to Charles J. Petry and assinged to the same assignee as the present invention. Both of these patents are incorporated herein by reference. Both patents concern a multiplicity of independently signalling thermal probes or sensors or detectors for the sensing of the level or depth or extent of the pool of molten metal in twin-belt continuous casting machines. These multiple probes are in bearing or skating contact with the reverse or water-cooled side of a thin flexible casting belt, which is normally of metal. If molten metal is touching the casting belt in an area on the front side of the belt at a point opposite the sensing probe, the probe becomes heated to a temperature as high as a difference of 90 degrees F. or 50 degrees C. (.DELTA.T) above the ambient temperature of the cool to tepid coolant water against the belt, though such heating is not instantaneous. A jacket of copper or other efficiently heat-conducting material is used to effect optimum transfer of heat to the thermal sensor within. In accord with the present invention, the probe has a flat-faced external shoe which is streamlined to minimize the disturbance to the flow of coolant. The probe should be flexibly mounted in a direction perpendicular to the belt, in order to maintain reliable and full bearing contact of its shoe against the reverse side of the casting belt. This flexible mounting may be accomplished notably by a suitably disposed helical spring or by a cantilever spring mount.
Three modes of pouring of molten metal are used in connection with twin-belt continuous casting machines: injection feeding (FIG. 13), closed-pool feeding (FIGS. 14 and 14A), and open-pool feeding (FIG. 15). The signal or information afforded by the above-mentioned thermal sensing probes has proved useful in the operation of twin-belt continuous casting machines, especially those operating under difficult conditions in all three pouring modes and most especially where optical means of detecting the level of the pool of molten metal within the mold have proven difficult or impossible. An optical system is described in U.S. Pat. No. 4,276,921 of Lemmens and Gielen.
We refer herein to the upper and lower casting belts. But in the case of a vertical caster, we mean simply the two belts or, again, in the case of a twin-belt wheel caster, the outer and inner belts. In many installations other than a twin-belt wheel caster, the two belts converge directly opposite each other as occurs around opposed upstream pulleys. This convergence defines the entrance or input region IR (FIG. 1) to the casting region. In such installations, molten metal M (FIG. 13) is usually fed into the casting machine through a close-fitting nosepiece (or "nozzle" or "snout") N (FIG. 13) which semi-seals the entrance to a clearance typically of 0.010 to 0.020 inch (0.25 to 0.50 mm) more or less, as is done in the casting of aluminum. When the casting space or mold cavity C within the casting machine is filled with molten or freezing metal thereby, the technique is called "injection feeding." This term is applied only to instances where the casting region of the machine is in this way entirely filled with freezing metal, with no void or gaseous space G above the metal inside. This injection feeding mode is illustrated in FIG. 13. The high surface tension notably of aluminum, and the tenacity of its oxide films, enable the pool of metal to fill up agains a not-too-thick nosepiece or nozzle N without backward leakage and consequent freezing into fins. Such congealing leakage would of course damage the nosepiece. In injection feeding, as shown in FIG. 13, control of the pool level within the mold cavity is by definition not applicable. But control of the level of the molten aluminum M in the large open tundish T (FIG. 13), which feeds the casting region C, is indeed critical, since too high a head there will cause high head within the mold region itself which is apt to cause finning through the gaps and damage to the nosepiece, thereby interrupting the entire continuous casting process up and down the line, forcing a restart of all operations from metal feeding to in-line rolling.
There are times when it may be well to create a smallish gas-filled void or cavity G (FIG. 14) inside the mold, above the pool P of molten metal M, in order (1) that the head of metal will not cause flashing of the metal under the metal-feeding nosepiece N and (2) in order that an inert atmosphere be assured to be in contact with the molten pool, as described in U.S. patent applications Ser. Nos. 372,459 dated Apr. 28, 1982, and 631,595 dated July 17, 1984. This cavity G may be desirable for instance in the continuous casting of a section with a substantial vertical thickness, like aluminum bar (as opposed to relatively thin slab). The pool P is maintained at a level below the point at which the void G would be replaced by molten metal. In this way, the molten metal M does not touch the full vertical height of the blunt exit end E of the nosepiece or snout. This technique is called "closed pool feeding" and is illustrated in FIG. 14. While the apparatus appears to suggest injection as in FIG. 13, the metal flowing immediately out from the nosepiece end E in closed-pool feeding encounters neither more molten metal nor the back pressure inherent in true injection feeding; hence, the term "injection" is not used herein for the closed-pool feeding technique.
In yet other twin-belt casting machine applications, as shown in FIG. 15, the lower (or inner) casting belt is so disposed or offset relative to the opposite or upper belt so as to support a free and open pool P of molten metal M. The metal M is introduced by means of a usually open-top runner Rn that is substantially smaller in cross section than the cross-sectional area of the casting region C between the casting belts. This is "open pool" feeding and is illustrated in FIG. 15. To permit easy pouring right in the pool P, the upper belt UB of an essentially horizontal straight caster is usually offset and made to converge toward the lower belt LB some distance downstream from where the lower belt leaves its upstream lower pulley ULP. This offset occurs when the upper carriage of such a machine is positioned a certain distance downstream. The offset may be varied. Open-pool pouring is to date the usual technique in the casting of copper or steel. Open-pool pouring is also used in the casting of lead, in which the problems of oxidation and cold shuts are not as serious as with aluminum.
The open-pool feeding arrangement (FIG. 15) is now used for continuous casting of metals of high melting point, such as copper and steel. An externally mounted telescopic optical sensor has been used to detect the visible or the infra-red radiation emanating from the free, open surface of the open metal pool within the mold; see U.S. Pat. No. 4,276,921 of Lemmens and Gielen, assigned to Metallurgie Hoboken-Overpelt of Belgium. The information from the optical sensor is used to control the rate of pouring so as to stabilize the open pool at the desired level.
However, the optical method is less appropriate in the casting of metals of lower melting point, such as lead, zinc, or perhaps aluminum, since the radiation is of diminished intensity, and oxide films may induce wide control-signal variations, notably with aluminum. Again, while the optical-sensing method works fairly well in the open-pool continuous casting of copper wire bar of 60.times.93 mm, the optical method becomes impractical for such casting of bar of narrow width, such as 50.times.58 mm copper bar, since the runner RN or spout which introduces the metal M into the mold area must occupy nearly all of the correspondingly narrow space at the entrance to the mold, thereby obstructing the optimum path of radiation to the externally mounted optical sensor. Moreover, smallish mold cavities that go with the casting of wire bar are more susceptible to internal reflections from edge-dam blocks, which reflections tend to confuse the sensing equipment. Careful aiming and adjustment of the normally employed zoom lens of the optical sensor may at times meet these problems. But the generally needed adjustments occurring from shift to shift have at times resulted in inconsistent casting machine operation.
A third problem applies to both the open-pool and closed-pool modes of pouring. In the earlier method of ascertaining the level of the pool of metal within the mold cavity by means of separately-indicating, multiple thermal probes, the indication of level was not contiuous but occurred in only a small number of discrete steps over the range of pool-height sensitivity. The probes responded with signals of essentially "yes" or "no". The number of steps corresponded to the necessarily limited number of thermal sensing probes, because the probes could, of necessity, be practically inserted only in particular locations due to the congested presence of other machine elements, notably backup rollers and water handling apparatus. The lack of a relatively continuous indication of pool level meant less information and less accurate level control when that multiple thermal probe apparatus was so used.
The belts of a twin-belt continuous metal casting machine are typically within the range of 0.025 to 0.078 of an inch (0.63 mm to 2 mm) in thickness, though the thickness is not necessarily confined to this range. Casting belts for wheel-and-belt casting machines, conventionally using only one casting belt, are apt to be appreciably thicker than this range includes.